A method has been developed to measure rapidly and accurately three important material constants of a hydrogel sample. The shear modulus, m, the bulk modulus, k, and the hydraulic permeability, l/f can be determined from a single stress-relaxation experiment. The material constants are treated as free parameters whose values are optimally estimated by minimizing the variance between predicted and empirical force relaxation waveforms. Values of the three constants obtained by this method agree with values obtained by independent free-swelling and permeability measurements.